Provided is a slide rail unit which, when accommodating a second rail (2) in a first rail (1), can automatically draw in the second rail to an end position and retain it at that position and which can be produced easily with a small number of parts and at low cost, the slide rail unit being equipped with a retaining pin (30) provided upright so as to be movable with respect to the first rail (1) in the longitudinal direction and the width direction thereof, an elastic member (31) for urging the retaining pin (30) toward the stroke end of the second rail (2), a pin guide member (32) provided on the first rail (1) and adapted to lock the retaining pin (30) at a standby position spaced apart from the stroke end against an urging force of the elastic member (31) and to guide the retaining pin (30) detached from the standby position toward the stroke end with the urging force, and a cam member (33) provided on the second rail (2) and adapted to detach the retaining pin (30) from the standby position of the pin guide member (32) and lock the retaining pin as it overlaps the pin guide member (32).